Dr Rajesh Bhagat is an interdisciplinary researcher working at the G. K. Batchelor Fluid Dynamics Laboratory at the Department of Applied Mathematics and Theoretical Physics (DAMTP) at the University of Cambridge. He became a Henslow Fellow in 2020. His research spans two distinct fields: interfacial flows and building ventilation flows, with a focus on indoor airborne disease transmission and sustainable living.
Throughout human history, nothing has killed more people than infectious diseases, a fact that has recently come to the fore with the Covid-19 pandemic. The pandemic highlighted the need for a better understanding of airborne disease transmission – which to a large extent is a fluid dynamics problem.
Bhagat’s work is enabling deeper understanding of airborne disease transmission, the processes by which droplets and aerosols – the carrier of the airborne pathogens – are formed in human vocal tract, and their subsequent transport in the indoor environment; in order to develop mitigating strategies and to reduce the risk of airborne transmission of diseases, not only for viruses like Covid-19, but also common airborne diseases such as flu and the common cold. It's no easy task however, especially with older buildings which would need retrofitting strategies to achieve the desired outcomes.
Bhagat is developing the mathematics necessary to understand the problems of droplets and aerosols formation of complex fluids such as saliva and other expiratory fluids in human vocal tract. He is also researching 'Displacement ventilation', a system which uses body heat to drive the flow of air up to be removed from openings in the upper level (celling), while heavy cold air enters from the lower lever openings (floor). This natural 'Displacement ventilation' also has the benefits of being net-zero, over energy-consuming mechanical ventilation systems.
In the lab at DAMTP, Bhagat conducts ventilation and interfacial flow experiments. The aim of these experiments is to develop the underlying mathematics necessary for a deeper understanding of airborne disease transmission and nonideal/practical building ventilation flows, which will enable the practical goals – development of easily implementable and sustainable ventilation methods and other strategies necessary for clean indoor environments.
It is established that the full-scale ventilation experiments, could be simulated at a smaller scale, in a water bath, using hot water or saline solutions, in a dynamically similar situation, without losing the key physics. But most importantly, this method allows us to prod, probe, examine and visualise the flow, enabling us to unravel the key physics. In one of the exemplar works, warm coloured dye solution and a heated manikin are used to replicate breath and heat emitting from the body. The dyes can be seen raising up into the lock up layer, before being removed from upper-level openings.
Many challenges remain, ranging from a person walking in a stratified room inducing mixing, to the ultimate fate of the mid-sized droplets which can potentially be aerosolised, as well as flow in the vicinity of openings. Furthermore, ventilation can’t be the silver bullet to all problems, for example wintertime ventilation is a concern which underscores the tension between ventilation needs and energy efficiency. Consequently, other solutions supplementing ventilation needs, such as filtration and UV deactivation of airborne diseases carriers is also necessary, all of which Bhagat is now investigating.
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The dynamics of infectious disease (ID) require fast accurate diagnosis for effective management and treatment. Without affordable, accessible diagnostics, syndromic or presumptive actions are often followed, where positive cases may go undetected in the community, or mistreated due to wrong diagnosis. In many low and middle income countries (LMICs), this undermines effective clinical decision-making and infectious disease containment.
Unsteady effects occur in many natural and technical flows, for example around flapping wings or during aircraft gust encounters. If the unsteadiness is large, the resulting forces can be quite considerable. However, the exact physical mechanisms underlying the generation of unsteady forces are complex and their accurate prediction remains challenging. One strategy is to identify the dominant effects and describe these with simple analytical models, first proposed a hundred years ago. When used successfully, this approach has the advantage that it also gives us a conceptual understanding of unsteady fluid mechanics.
In this lecture I will explain some of these ideas and demonstrate how they can still be useful today. As a practical example, I will show how the forces experienced in a wing-gust encounter can be predicted – and how the predictions can be used to mitigate the gust effects. The lecture will be illustrated with images and videos from simple, canonical, experiments.
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